A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Due to the demands in lithography to obtain patterns having smaller and smaller structures, many developments take place to be able to fulfil these requirements. One of these developments is, for example, the use of diffraction patterns, thereby making use of optical diffraction when projecting a pattern onto the substrate. To be able to make use of diffraction phenomena, a radiation beam from a light source may be projected onto the patterning device under an oblique angle, i.e. under an angle, which differs from 90 degrees with respect to a surface of the patterning device. In order to obtain such oblique incident radiation beam, use may be made of, for example, a mirror or mirror assemblies.